Nitrox: boat fill vs. membrane (shop) fill

Hi fellow divers,

Can anyone explain the difference in Nitrox fills? I have 02 clean tanks which I thought was a good idea at the time, for local diving. However, a few boats can fill Nitrox but I'm told that means if they do that tank will not be fillable through the dive shop. The boats have ? is it blended or unblended? What's the difference between blended and membrane fills?

I have read about it online, asked a few people about it - but for some reason, it's just not completely clear to me. Can you help?

Thanks,
R

The difference to you is zilch. The manner in which the blender makes their nitrox will dictate the needs for tank maintenance.

For blenders who have a membrane system, the way the nitrox is made is effectively by pushing air through a filter that removes nitrogen (thus enriching the output with higher concentrations of oxygen). The gas is then compressed into your tank. Because your tank is never exposed to pure oxygen, it does not need to be O2 clean. Membrane systems have the advantage of not requiring that kind of tank maintenance but are limited in how high a percentage of nitrox they can manufacture and are also generally the most costly way to produce it.

Other blenders use what is called a "blending stick". Which is effectively a tube at ambient pressure with multiple gas inlets for oxygen, air, etc. Gas is released into the stick and the mix controlled with "flow meters" that combine to create the desired nitrox. The stick has an outlet that is run to a compressor which is used to pump up your tank. Here again, the tank is never interfaced with pure oxygen and doesn't require special handling.

Basic blending uses a procedure called "partial pressure" where gases are decanted directly into your tank; generally oxygen first and then compressed air to top off. Because this procedure begins with a tank being attached to a source of pure oxygen, the tank should be specially cleaned and preserved (there is much to discuss on the feasibility of how clean a tank can be kept, but that's for another discussion).

Some blenders will partial pressure blend their own storage tanks with nitrox. In that case, their storage tanks need to be oxygen cleaned but your tank does not (since they will decant pre-mixed nitrox from their clean tanks into your tank, it never comes into contact with O2)

In California we have boats that do all of the above, so it matters when you inquire about Nitrox to ask how they're making it (membrane, continuous blending, banked or partial pressure). Which boat/operator are you referring to?

The boat is probably filling with a blending stick, and is not using hyperfilters, so the gas being put in your tank is not O2 clean, and as such after getting a fill on the boat, your tank will no longer be O2 clean.

The shop is probably doing partial pressure blending where they add pure O2 and then top off with air (run through hyperfilters). In order to add the pure O2, the tank needs to be O2 clean.

Using a membrane system does not require a tank to be O2 clean (much like using a blending stick), although if the gas is run through hyperfilters, then the cylinders will still be O2 clean if they were before (again, just like with a blending stick).

nimoh:

The boat is probably filling with a blending stick, and is not using hyperfilters, so the gas being put in your tank is not O2 clean, and as such after getting a fill on the boat, your tank will no longer be O2 clean.

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Some boats here do this with hyperfilters inline. The Marissa in San Diego for instance.

The shop is probably doing partial pressure blending where they add pure O2 and then top off with air (run through hyperfilters). In order to add the pure O2, the tank needs to be O2 clean.

Click to expand...

This is common but generally is banked at most of the shops I'm aware of due to volume on the weekends and/or to support classes.

Using a membrane system does not require a tank to be O2 clean (much like using a blending stick), although if the gas is run through hyperfilters, then the cylinders will still be O2 clean if they were before (again, just like with a blending stick).

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For nitrox, If your shop is requiring that you have an O2 clean tank, I'd honestly find another shop. Too much of a pain in the arse and there are too many local alternatives.

mathauck0814:

Some boats here do this with hyperfilters inline. The Marissa in San Diego for instance.

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The OP said that if he got the fill on the boat then he couldn't get a fill at the shop...and I am guessing that it is because of not using hyperfilters on the boat and partial pressure blending at the shop.

That doesn't mean that using hyperfilters on a boat isn't possible.

mathauck0814:

This is common but generally is banked at most of the shops I'm aware of due to volume on the weekends and/or to support classes.

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If you accept the previous sentence, then the shop is not using banked nitrox (or they have a weird requirement for tanks to be O2 clean to fill with banked nitrox).

mathauck0814:

For nitrox, If your shop is requiring that you have an O2 clean tank, I'd honestly find another shop. Too much of a pain in the arse and there are too many local alternatives.

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Can't say I completely disagree, assuming there are reasonable alternatives. I was merely trying to help with the OP's question and don't know much about the dive shop scene in Orange County CA.

ShootingUnderwater.com:

Hi fellow divers,

Can anyone explain the difference in Nitrox fills? I have 02 clean tanks which I thought was a good idea at the time, for local diving. However, a few boats can fill Nitrox but I'm told that means if they do that tank will not be fillable through the dive shop. The boats have ? is it blended or unblended? What's the difference between blended and membrane fills?

I have read about it online, asked a few people about it - but for some reason, it's just not completely clear to me. Can you help?

Thanks,
R

Click to expand...

Boats are far more likely to have a membrane or nitrox stick than a shop. Boats need a lot of nitrox and they need it right now, shops have all day to pp fill. With that said, a boat doesn't necessarily care about tanks clean enough for O2, they just need something that will handle 40%.

nimoh:

The boat is probably filling with a blending stick, and is not using hyperfilters, so the gas being put in your tank is not O2 clean, and as such after getting a fill on the boat, your tank will no longer be O2 clean.

The shop is probably doing partial pressure blending where they add pure O2 and then top off with air (run through hyperfilters). In order to add the pure O2, the tank needs to be O2 clean.

Using a membrane system does not require a tank to be O2 clean (much like using a blending stick), although if the gas is run through hyperfilters, then the cylinders will still be O2 clean if they were before (again, just like with a blending stick).

Click to expand...

Yeah, what you said, except some of us keep our systems in good enough shape to consistently make OCA without needing hyper filters.

Wookie:

Boats are far more likely to have a membrane or nitrox stick than a shop. Boats need a lot of nitrox and they need it right now, shops have all day to pp fill. With that said, a boat doesn't necessarily care about tanks clean enough for O2, they just need something that will handle 40%.



Yeah, what you said, except some of us keep our systems in good enough shape to consistently make OCA without needing hyper filters.

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yes, like I said, I was only making a guess based on what the OP explained.

disclaimer:
I was in no way trying to suggest that this is how all boats operate, or how all shops operate but only trying to suggest a scenario that might explain the OP's situation. I apologize to all boat operators for painting them in a bad light by suggesting that one boat operator may or may not filling tanks with gas that is not O2 clean

OP - Since I do know the area and the boats and shops, it would be helpful to know which boats/shops you're referring to so that we can help steer you toward others that have less onerous requirements.

I would be surprised if a reasonably busy shop is PP filling recreational nitrox. It would seem to be a labour intensive and more risky approach than simply banking.

Hello OP,

The boats I dive from use a Nuvair type system (membrane filter system that supplies O2 enriched air to the HP compressor's intake (Vision, Conception, Truth, Peace, Marissa, Lois Anne).

Dolphin Dive shop requires O2 clean tanks because they bank 50% Nitrox. They use the 50% Nitrox to PP blend down to less than 40%. Dolphin does not require me to have a tank O2 cleaned again, after my 02 clean tanks have been filled with a Nuvair type system. Air is a different story if they don't know the source. Air from one of the boats listed above seems to be OK (I guess the compressors from these boats are oil free? Dunn-no! They are the same compressors used to fill Nitrox--the intake either receives ambient air or EANX Nuvair processed gas).

It is still a little confusing as there seems to be different standards, or I don't quite understand it.

markm